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1. A Series of Zinc Mononuclear Complexes with Imidoyl Amidine Ligands: Syntheses, Crystal Structures, and Photoluminescence Properties

   https://doi.org/10.3390/chemistry6040045  

   Nijhuis, Mart Ruben; Yamba, Zaina; Novikov, Egor; Fonari, Marina S; Timofeeva, Tatiana V; Castañeda, Raúl (August 2024, Chemistry)

   Three amidine-based ligands were used in the crystal design of a series of mononuclear Zn(II) complexes. Interaction of zinc chloride, ZnCl2, with N-2-pyridylimidoyl-2-pyridylamidine (Py2ImAm) resulted in complexes [Zn(Py2ImAm)2] (1) and [ZnCl2(Py2ImAm)] (2). In [Zn(Py2ImAm)2] (1, monoclinic, P21/c), the metal ion was coordinated with the bidentate pocket of the anionic form of Py2ImAm, while in [ZnCl2(Py2ImAm)] (2, monoclinic, P21/n), the tridentate coordination to a neutral Py2ImAm was completed by two chloride anions. This structural variation was achieved by a pH-controlling strategy using the weak base triethylamine (TEA). Otherwise, three ionic complexes were obtained with 2-amidinopyridine (PyAm) and Zinc(II), [ZnCl(PyAm)2]Cl (3, triclinic, P-1), [ZnCl(PyAm)2]2[ZnCl4]·C2H5OH (4, monoclinic, P21/n), and [ZnCl(PyAm)2]2Cl·CH3OH (5, triclinic, P-1). They comprised the same [ZnCl(PyAm)2]+ monocation with a butterfly-like shape provided by the bidentate chelate coordination of two PyAm neutral entities and a chloride ligand. In a similar butterfly shape, ionic complex [ZnCl(PmAm)2]2[ZnCl4] (6, monoclinic, C2/c) comprised the mononuclear [ZnCl(PmAm)2]+ cations with two bidentate chelate-coordinated 2-amidinopyrimidine (PmAm) as neutral ligands. The Zn(II) pentacoordinated arrangement in 3–6 was variable, from square pyramidal to trigonal bipyramidal. The reported compounds’ synthetic protocols, crystal structures and photoluminescence properties are discussed. 

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2. Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

   https://doi.org/10.1039/C9OB00478E  

   Marichev, Kostiantyn O.; Doyle, Michael P. (April 2019, Organic & Biomolecular Chemistry)

   This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor–acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity. 

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3. A Redox‐Active Tetrazine‐Based Pincer Ligand for the Reduction of N‐Oxyanions Using a Redox‐Inert Metal

   https://doi.org/10.1002/chem.202101302  

   Beagan, Daniel_M; Maciulis, Nicholas_A; Pink, Maren; Carta, Veronica; Huerfano, I_J; Chen, Chun‐Hsing; Caulton, Kenneth_G (July 2021, Chemistry – A European Journal)

   Abstract The reaction chemistry of the bis‐tetrazinyl pyridine ligand (btzp) towards nitrogen oxyanions coordinated to zinc is studied in order to explore the reduction of the NO_x⁻substrates with a redox‐active ligand in the absence of redox activity at the metal. Following syntheses and characterization of (btzp)ZnX₂for X=Cl, NO₃and NO₂, featuring O−Zn linkage of both nitrogen oxyanions, it is shown that a silylating agent selectively delivers silyl substituents to tetrazine nitrogens, without reductive deoxygenation of NO_x⁻¹. A new synthesis of the highly hydrogenated H₄btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H₄btzp)ZnX₂for X=Cl and NO₃, both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H₄btzp)ZnCl₂complex does not bind zinc in the pincer pocket, but instead H₄btzp becomes a bridge between neighboring atoms through tetrazine nitrogen atoms, forming a polymeric chain. The reaction of AgNO₂with (H₄btzp)ZnCl₂is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H₄btzp reducing equivalents form Ag⁰and thus the chloride ligand remains coordinated to the zinc metal center to yield (btzp)ZnCl₂. To compare with AgNO₂, experiments of (H₄btzp)ZnCl₂with NaNO₂result in salt metathesis between chloride and nitrite, highlighting the importance of a redox‐active cation in the reduction of nitrite to NO. 

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4. Distal scaffold flexibility accelerates ligand substitution kinetics in manganese( i ) tricarbonyls: flexible thianthrene versus rigid anthracene scaffolds

   https://doi.org/10.1039/D2DT04048D  

   Labrecque, Jordan; Cho, Yae-In; McIntosh, Daniel K.; Agboola, Faridat; Rose, Michael J. (March 2023, Dalton Transactions)

   This work investigates the effect of molecular flexibility on fundamental ligand substitution kinetics in a pair of manganese( i ) carbonyls supported by scaffold-based ligands. In previous work, we reported that the planar and rigid, anthracene-based scaffold with two pyridine ‘arms’ ( Anth-py 2 , 2) serves as a bidentate, cis donor set, akin to a strained bipyridine (bpy). In the present work, we have installed a more flexible and dynamic scaffold in the form of thianthrene ( Thianth-py 2 , 1), wherein the scaffold in the free ligand exhibits a ∼130° dihedral angle in the solid state. Thianth-py 2 also exhibits greater flexibility (molecular motion) in solution compared with Anth-py 2 , as evidenced by longer 1 H NMR T 1 times Thianthy-py 2 ( T 1 = 2.97 s) versus Anth-py 2 ( T 1 = 1.91 s). Despite the exchange of rigid Anth-py2 for flexible Thianth-py2 in the complexes [( Anth-py 2 )Mn(CO) 3 Br] (4) and [( Thianth-py 2 )Mn(CO) 3 Br] (3), respectively, nearly identical electronic structures and electron densities were observed at the Mn center: the IR of 3 exhibits features at 2026, 1938 and 1900 cm −1 , nearly identical to the features of the anthracene-based congener (4) at 2027, 1936 and 1888 cm −1 . Most importantly, we assessed the effect of ligand-scaffold flexibility on reactivity and measured the rates of an elementary ligand substitution reaction. For ease of IR study, the corresponding halide-abstracted, nitrile-bound (PhCN) cations [( Thianth-py 2 )Mn(CO) 3 (PhCN)](BF 4 ) (6) and [( Anth-py 2 )Mn(CO) 3 (PhCN)](BF 4 ) (8) were generated in situ , and the PhCN → Br – back-reaction was monitored. The more flexible 3 (thianth-based) exhibited ∼3–4× faster ligand substitution kinetics ( k 25 C = 22 × 10 −2 min −1 , k 0 C = 43 × 10 −3 min −1 ) than the rigid analogue 4 (anth-based: ( k 25 C = 6.0 × 10 −2 min −1 , k 0 C = 9.0 × 10 −3 min −1 ) on all counts. Constrained angle DFT calculations revealed that despite large changes in the thianthrene scaffold dihedral angle, the bond metrics of 3 about the metal center remain unchanged; i.e. the ‘flapping’ motion is strictly a second coordination sphere effect. These results suggest that the local environment of molecular flexibility plays a key role in determining reactivity at the metal center, which has essential implications for understanding the reactivity of organometallic catalysts and metalloenzyme active sites. We propose that this molecular flexibility component of reactivity can be considered a thematic ‘third coordination sphere’ that dictates metal structure and function. 

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5. Photoinduced Electron Transfer in Axially Coordinated Supramolecular Zinc Tetrapyrrole Bis(styryl)BODIPY Donor‐Acceptor Conjugates

   https://doi.org/10.1002/cptc.202000221  

   Benitz, Alejandro; Thomas, Michael_B; de_Silva, Imesha; Nesterov, Vladimir_N; Verbeck, IV, Guido_F; D'Souza, Francis (November 2020, ChemPhotoChem)

   Abstract Photoinduced electron transfer (PET) in newly assembled dyads formedviametal‐ligand axial coordination of phenylimidazole‐functionalized bis(styryl)BODIPY (BODIPY(Im)₂) and zinc tetrapyrroles, that is, zinc tetratolylporphyrin (ZnP), zinc tetra‐t‐butyl phthalocyanine (ZnPc) and zinc tetra‐t‐butyl naphthalocyanine (ZnNc), in non‐coordinatingo‐dichlorobenzene (DCB) is investigated using both steady‐state and time‐resolved transient absorption techniques. The structure of the BODIPY(Im)₂was identified by using single crystal X‐ray structural analysis. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational and electrochemical methods. The binding constants measured from optical absorption spectral studies were in the range of ∼10⁴ M⁻¹for the first zinc tetrapyrrole binding and suggested that the two imidazole entities of bis(styryl)BODIPY behave independently in the binding process. The energy level diagram established using spectral and electrochemical studies suggested PET to be thermodynamically unfavorable in the ZnP‐bearing complex while for ZnPc‐ and ZnNc‐bearing complexes such a process is possible when zinc tetrapyrrole is selectively excited. Consequently, occurrence of efficient PET in the latter two dyads was possible to establish from femtosecond transient absorption studies wherein the electron transfer products, that is, the radical cation of zinc tetrapyrrole and the radical anion of BODIPY(Im)₂, was possible to spectrally identify. From target analysis of the transient data, time constants of circa 3 ns for ZnPc^⋅+:BODIPY⋅⁻and circa 0.5 ns for ZnNc⋅⁺:BODIPY⋅⁻were obtained indicating persistence of the radical ion‐pair to some extent. The electron acceptor property of bis(styryl)BODIPY in donor‐acceptor conjugates is borne out from the present study. 

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